Barrier height voltage reference

ABSTRACT

A barrier height voltage reference includes two field-effect transistors which are substantially identical except for their gate-to-channel potential barrier characteristics and which are biased to carry equal drain currents at equal drain voltages. The resulting difference in potential between the gate contacts of the two field effect transistors produces a voltage reference which is substantially independent of operating point, supply potential, and temperature.

CROSS REFERENCE TO RELATED APPLICATION

This is a continuation in part of copending U.S. patent application Ser. No. 587,188 filed June 16, 1975 titled "Flat-Band Voltage Reference" by Morley C. Tobey, Jr., et al. now U.S. Pat. No. 3,975,648 issued Aug. 17, 1976.

BACKGROUND AND SUMMARY OF THE INVENTION

Typical solid-state voltage reference devices generally obtain a reference voltage from a predictable breakdown voltage or from a known forward biased current-voltage relationship of a junction device. Temperature dependence of one device is often compensated for by combination with another device also having a temperature dependence. A constant-current source is usually required for the best precision. The present invention is a voltage reference comprising a pair of field-effect devices each substantially identical to the other except for gate-to-channel potential barrier characteristics. Each field-effect device is biased with a substantially identical drain current to produce a reference voltage corresponding to the difference in barrier potentials at the gate-to-channel interface of the two field-effect devices. The invention provides a voltage reference which is substantially independent of operating point, supply potential, and temperature.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional drawing of a junction field-effect transistor.

FIGS. 2A, 2B and 2C show energy band diagrams of two substantially identical semiconductor substrates wherein the first has in combination therewith, a metal gate of a first type and the second substrate has in combination therewith, a metal gate of a second type.

FIGS. 3A, 3B and 3C show energy band diagrams of two semiconductor substrates having gates of the same material as the substrates, the gates being doped with different concentrations and forming P-N junctions with their respective substrates.

FIGS. 4A, 4B and 4C show energy band diagrams of a first semiconductor substrate of a first type and a second semiconductor substrate substantially identical to the first semiconductor substrate, the first semiconductor substrate having a semiconductor gate of a second type and the second semiconductor substrate having a semiconductor gate of a third type.

FIG. 5 is a diagram of a voltage reference made in accordance with the invention.

FIGS. 6A, 6B, 6C and 6D are diagrams illustrating the processing steps for fabricating a voltage reference monolithically in accordance with one embodiment of the invention.

FIG. 7 is a diagram of another voltage reference made in accordance with the invention.

FIG. 8 is a diagram showing a method for combining a plurality of voltage references made in accordance with the invention to achieve larger reference voltages.

FIG. 9 shows a preferred voltage reference wherein surface area required for integrated fabrication is minimized.

FIG. 10 is a diagram of a preferred embodiment utilizing voltage variable capacitors in accordance with the invention.

FIG. 11 is a diagram of another preferred embodiment utilizing voltage variable capacitors.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The electrical characteristics of an N-channel field effect transistor, FET hereafter, operated above turn-on and below pinch-off are described, for example by A. S. Grove, Physics and Technology of Semiconductor Devices, John Wiley and Sons, Inc., New York, 1967: ##EQU1## where I_(D) is the drain current, Z is the channel width, q is the magnitude of the electronic charge, μ_(n) is the electron mobility, N_(D) is the density of donors in the channel region, d is the channel thickness, L is the channel length, V_(D) is the drain voltage which is measured between the drain and the source, ε_(s) is the semiconductor dielectric constant, φ_(B) is the built-in potential between the gate and the channel and referred to herein as the barrier potential, and V_(G) is the gate voltage which is measured between the gate and the source.

A FET operated in the pinch-off condition is also described by Equation 1 wherein the value for V_(D) is constant and V_(D) is given by (see, for example, Physics and Technology of Semiconductor Devices as cited above): ##EQU2##

Equations 1 and 2 are applicable to a junction FET having geometry similar to that of FIG. 1. Equations similar to equations 1 and 2 are also applicable to metal gate FETs as well as to heterojunction FETs.

For two FETs, identical except for their gate-to-channel potentials and biased with equal I_(D) 's in pinch-off or equal I_(D) 's and V_(D) 's below pinch-off, the quantity (φ_(B) - V_(G)) is identical. A reference voltage can therefore be developed, which is:

    V.sub.Ref = Δ V.sub.G = Δ φ.sub.B          Equation 3

where V_(Ref) is the reference voltage, Δ V_(G) is the difference in gate voltages, and Δ φ_(B) is the difference in barrier potentials. This reference voltage is developed whether or not the FET's are operated above or below pinch-off. It is also developed when the FET's are operated below turn-on so long as identical I_(D) 's and V_(D) 's are maintained.

Referring to FIG. 1, there is shown an FET having an n-channel 802 connected to a back gate 804 of p-type semiconductor material, a front gate 806 also of p-type semiconductor material, a source contact 808, and a drain contact 810. While FIG. 1 shows a junction FET having both a top gate 806 and a back gate 804 wherein both form step homojunctions, the only requirement for making a voltage reference in accordance with this invention is that two FET's be substantially identical except for the gate-to-channel built-in potential of at least one gate-to-channel junction.

A difference in φ_(B) 's is obtained, for example, by utilizing two different metals for gates which make rectifying contacts to the substrates but which result in different gate-to-channel built-in potentials. FIG. 2(a) shows the energy band diagrams of two substantially identical semiconductor substrates 402 and 404 which are, for example, n-type, the first semiconductor substrate 402 being connected to a first gate metal 406, and the second semiconductor substrate 404 being connected to a second gate metal 408. Gate metals 406 and 408 are chosen so as to make rectifying contact with the semiconductor substrates 402 and 404 and so that the barrier potential φ_(B1) of the first FET is greater than the barrier potential φ_(B2) of the second FET. Since φ_(B2) is smaller than φ_(B1') the channel width is larger in the second semiconductor substrate 404 than in the first semiconductor substrate 402. As shown in FIG. 2(b) biasing the second gate metal 408 with respect to the second semiconductor substrate 404 by an amount φ_(B2) - φ_(B1) results in the channel width of the second semiconductor substrate 404 being the same as in the first semiconductor substrate 402. A reference voltage V_(Ref) = φ_(B2) - φ_(B1) is thereby produced. In FIG. 2(c) gate metals 406 and 408 are biased by different voltages from those shown in FIG. 2(b). However, the channel width in the second semiconductor substrate 404 is the same as in the first semiconductor substrate 402, resulting in the same reference voltage (V_(Ref) = φ_(B2) - φ_(B1)) as in FIG. 2(b).

The difference in φ_(B) 's is also obtained by utilizing two semiconductor materials for the gates which are the same as the semiconductor material of the two substrates and utilizing a first gate semiconductor material which has a first doping density different from the doping density of the second gate semiconductor material. FIG. 3(a) illustrates an energy band diagram for two substantially identical semiconductor substrates 502 and 504. The first semiconductor substrate 502 is connected to a first semiconductor gate 506, and the second semiconductor substrate 504 is connected to a second semiconductor gate 508. The doping species of semiconductor gates 506 and 508 are chosen so that the majority carrier of the semiconductor substrates 502 and 504 is of different polarity from that of the semiconductor gates 506 and 508. The magnitude of the doping density in the first semiconductor gate 506 is different from that of the second semiconductor gate 508 thereby producing different gate-to-channel built-in potentials. Since φ_(B2) is smaller than φ_(B1), the channel width is larger in the second semiconductor substrate 504 than in the first semiconductor substrate 502. As shown in FIG. 3(b), biasing the second semiconductor gate 508 with respect to the second semiconductor substrate 504 by an amount φ_(B2) - φ_(B1) results in the channel width of the second semiconductor substrate 504 being the same as in the first semiconductor substrate 502. A reference voltage (V_(Ref) = φ_(B2) - φ_(B1)) is thereby produced. In FIG. 3(c) semiconductor gates 506 and 508 are biased by different voltages from those shown in FIG. 3(b). However, the channel width in semiconductor substrate 504 is also the same as in the semiconductor substrate 502, resulting in the same reference voltage (V_(Ref) = φ_(B2) - φ_(B1)) as in FIG. 3(b).

The difference in φ_(B) 's is also obtained by utilizing two different semiconductor materials for the gates. FIG. 4(a) shows two substantially identical semiconductor substrates 602 and 604 which are of a first semiconductor material which is, for example, n-type. The first semiconductor substrate 602 is connected to a first semiconductor gate 606, which is of a second semiconductor material and which is, for example, p-type. The second semiconductor substrate 604 is connected to a second semiconductor gate 608 which is of a third semiconductor material and which is, for example, p-type. The materials and doping species of semiconductor gates 606 and 608 are chosen so that rectifying contact is made to the semiconductor substrates 602 and 604 and so that φ_(B1) is not equal to φ_(B2). Since φ_(B2) is smaller than φ_(B1), the channel width is larger in the second semiconductor substrate 604 than in the first semiconductor substrate 602. Biasing the second semiconductor gate 608 with respect to the second semiconductor substrate 604, as shown in FIG. 4(b) by the amount φ_(B2) - φ_(B1) results in the channel width of the second semiconductor substrate 604 being the same as in the first semiconductor substrate 602. A reference voltage V_(Ref) = φ_(B2) - φ_(B1) is thereby produced between the second semiconductor gate 608 and the first semiconductor gate 606. In FIG. 4(c) semiconductor gates 606 and 608 are biased by different voltages from those shown in FIG. 4(b). However, the channel width in the first semiconductor substrate 602 is also the same as in the second semiconductor substrate 604, resulting in the same reference voltage (V_(Ref) = φ_(B2) - φ_(B1)) as in FIG. 4(b).

Combinations of gate materials shown in FIGS. 2-4 can also be used to fabricate a voltage reference in accordance with this invention. Such voltage references include the following combinations: (1) a homojunction and a schottky barrier, (2) a homojunction and a heterojunction, and (3) a schottky barrier and a heterojunction.

Referring to FIG. 5, there is shown a first preferred embodiment made in accordance with the invention. A first n-channel FET 5 and a second n-channel FET 7 are substantially identical except for different barrier potentials. A bias supply 1 is connected between source contact 13 and drain contact 17 of FET 5 and a bias supply 3 is connected between source contact 15 and drain contact 19 of FET 7. The potentials of gate contact 35 of FET 5 and gate contact 37 of FET 7 are adjusted by gate biasing means not shown in FIG. 5. The gate biasing means not shown in FIG. 5, the supply source 1 and the supply source 3 are adjusted so that the drain voltage of FET 5 is substantially equal to the drain voltage of FET 7 and the drain current of FET 5 is substantially equal to the drain current of FET 7. A reference voltage, V_(Ref), is produced by the difference between the potentials of the gate contact 35 of FET 5 and the gate contact 37 of FET 7.

The matching of the two FET's is enhanced by fabricating them in close proximity to each other on the same substrate. The substrate doping densities and the mobilities are then substantially the same in both FET's. FIG. 6 shows processing steps to fabricate monolithically two schottky barrier FET's which are identical except for their gate metals. Referring to FIG. 6(a), a high resistivity substrate 702, such as, for example, silicon, is first cleaned and polished. A low resistivity epitaxial layer 704 is then grown on the substrate 702. The field oxide 706 is grown, and the isolation diffusion windows 708 opened, followed by the isolation diffusion 710. As shown by FIG. 6(b), source and drain windows 712 and 714 are etched, followed by source and drain diffusions 716 and 718. In FIG. 6(c) gate window 720 and source and drain contacts 722 and 724 are etched, followed by deposition of a first metal 725, as, for example, by RF sputtering platinum. The first metal is removed except over the source and drain contacts 722 and 724 and over gate window 720. Referring to FIG. 6(d), second gate window 726 is etched and a second metal 727 is deposited, as, for example, by electron-beam evaporating titanium. The second metal 727 is removed everywhere except over the previously deposited first metal 725, except for any necessary interconnecting metal and bonding pads, and except over the second gate window 726. Modifications to the above processing steps are possible. One such modification, for example, is to fabricate the two FET's either on the same or different substrates using the same processing steps until after the contact mask step. The FET's are separated and different gate field plate metals and interconnecting metals are deposited on each.

V_(Ref) is temparature dependent only to the extent of mismatches in the FET's and to the extent of the difference in temperature coefficients of their respective gate-to-channel barrier heights.

Referring to FIG. 7, there is shown another preferred embodiment made in accordance with the invention. A first p-channel FET 2 and a second p-channel FET 4 are substantially identical except for different gate-to-channel barrier heights. An amplifier 6 is coupled to drive gate 44 of FET 4. Amplifier 6 has a non-inverting input connected to source contact 20 of FET 2 and a resistor 8 and an inverting input connected to source contact 40 of FET 4 and a resistor 10. Amplifier 6 has a larger input impedance as compared with the resistances of resistors 8 and 10. A negative supply voltage is coupled across terminal 12 and ground 14. Drain contacts 22 and 42 are connected to terminal 12. Gate contact 24 of FET 2, resistor 8 and resistor 10 are connected to ground 14. The drain currents and drain voltages of FET 2 and FET 4 are maintained substantially equal by selecting matching resistances 8 and 10 and by the action of amplifier 6 driving the gate contact 44 of FET 4. A reference voltage is obtained across terminals 16 and 14.

Referring to FIG. 8, there is shown a combination comprising two of the voltage references depicted in FIG. 7. The reference voltage is obtained across terminals 16 and 14. A plurality of references may be similarly combined to provide larger reference voltages.

Referring to FIG. 9, matched FET's 28 and 30 are substituted for the matched resistors 8 and 10 in the embodiment depicted in FIG. 7. This substitution allows conservation of substrate area should large resistance values be required for the resistors R8 and R10.

Other φ_(B) dependent parameters, as, for example, the AC (alternating current) conductance and transconductance, of the FETs can also be detected and used to adjust the bias conditions so that a reference voltage is obtained.

The invention may also be fabricated using two voltage variable capacitors. Gate-to-substrate junctions are made, as described above for the FETs. Referring to FIG. 2, it is seen that the depletion layer in semiconductor substrate 404 is substantially the same as in semiconductor substrate 402 if gate 408 is biased with respect to semiconductor 404 at a voltage V_(Ref) more negative than gate 406 is biased with respect to semiconductor substrate 402. For such a bias condition the capacitance between gate 406 and substrate 402 will be substantially equal to that between gate 408 and substrate 404. The semiconductor junctions of FIG. 3 and the heterojunctions of FIG. 4 may also be used so long as the major part of the depletion layer extends into the substrates.

Referring to FIG. 10, a preferred embodiment made in accordance with the invention using voltage variable capacitors is shown. Voltages are applied to each of the two voltage variable capacitors so that the same depletion layer width is obtained in each voltage variable capacitor, thereby resulting in the same capacitance for each capacitor. The two capacitors are essentially identical except for their built-in potential. A reference voltage V_(Ref) is produced, which is given by:

    |V.sub.Ref |=|V.sub.G1 - V.sub.G2 |=|φ.sub.B1 - φ.sub.B2 |Equation 4

where V_(G1) and V_(G2) are gate-to-substrate voltages for first and second voltage-variable capacitors 202 and 204 and φ_(B1) and φ_(B2) are the respective built-in potentials for the first and second voltage variable capacitors 202 and 204. The gate-to-substrate voltages are varied to obtain matching capacitances.

An alternating current (AC) voltage source 206 is coupled by coupling capacitors 208 and 209 to gates 210 and 212 of the voltage variable capacitors 202 and 204, respectively. Substrate contacts 214 and 216 of the voltage variable capacitors 202 and 204 are coupled to ground 200 by resistors 218 and 219, respectively. Resistors 218 and 219 are of substantially identical resistance and are coupled respectively to rectifiers and filters 220 and 222. Outputs 228 and 230 of the rectifiers and filters 220 and 222 are coupled to differential inputs of amplifier 224. The amplifier 224 has an output coupled to the gate 210 of the voltage variable capacitor 202.

The resistance of resistors 218 and 219 is selected to be less than the magnitude of the reactance of voltage variable capacitors 202 and 204. Biasing means 226 adjusts the capacitance of voltage variable capacitor 204 to a convenient value and biasing means 226 has an impedance of magnitude greater than the magnitude of the reactance of voltage variable capacitor 204. The magnitudes of reactances of coupling capacitors 208 and 209 are chosen to be less than the magnitude of reactances of voltage variable capacitors 202 and 204 at the frequency of AC voltage source 206.

Alternating current voltages having magnitudes substantially proportional to the capacitances of voltage variable capacitors 202 and 204 are developed at substrate contacts 214 and 216. These voltages are converted to direct current (DC) voltages by rectifiers and filters 220 and 222 to produce signals at outputs 228 and 230 which drive differential amplifier 224. Amplifier 224 drives the voltage at the gate 210 of voltage variable capacitor 202 until the capacitance of voltage variable capacitor 202 is substantially equal to the capacitance of voltage capacitor 204. The reference voltage V_(Ref) is developed between gate 210 and gate 212 and is substantially equal to the difference in built-in potentials of field-effect capacitors 202 and 204.

Referring to FIG. 11, there is shown another preferred embodiment made in accordance with the invention. Identical AC current sources 305 and 306 drive identical AC currents into voltage variable capacitors 302 and 304 which are substantially identical except for their built-in potentials. Rectifying and filtering means 320 and 322 are connected to voltage variable capacitors 302 and 304 respectively.

A DC voltage is produced at input terminals 328 and 330 of amplifier 324 which forces the output of amplifier 324 to bias voltage variable capacitor 302 such that its capacitance is equal to that of voltage variable capacitor 304. A reference voltage V_(Ref) is produced between terminals 350 and 352 which is equal to the difference in the built-in potentials of voltage variable capacitors 302 and 304 (see Equation 4). Bias source 326 is adjusted to obtain a convenient value of capacitance.

Differences in voltage variable capacitor built-in potentials are produced, for example, by the techniques disclosed herein with respect to field-effect transistors.

Given below are tables of approximate voltage reference values produced by the embodiments discussed hereinabove for various metal combinations on the substrate material types indicated. In general, the values given below are processing dependent and are affected by the degree of substrate doping as well as species. A V_(Ref) obtained in practise can therefor vary from the V_(Ref) 's given below.

                  TABLE I                                                          ______________________________________                                         Substrate: n-type AlAs                                                                       Au       Pt                                                      ______________________________________                                         Au             --          0.2                                                 Pt             --          --                                                  ______________________________________                                    

                  TABLE II                                                         ______________________________________                                         Substrate: n-CdS                                                               Pt     Au       Pd       Cu     Ag      Ni                                     Pt  --     0.4      0.5    0.6    0.8     0.4                                  Au  --     --       0.06   0.18   0.3     0.3                                  Pd  --     --       --     0.12   0.2     0.2                                  Cu  --     --       --     --      0.06    0.09                                Ag  --     --       --     --     --       0.11                                ______________________________________                                    

                  TABLE III                                                        ______________________________________                                         Substrate: n-type CdSe                                                                     Pt       Au      Ag      Cu                                        Pt            --         0.12    0.06  0.04                                    Au            --         --      0.06  0.16                                    Ag            --         --      --    0.10                                    ______________________________________                                    

                  TABLE IV                                                         ______________________________________                                         Substrate: n-type CdTe                                                                     Au       Pt      Ag      Al                                        Au            --         0.02    0.06  0.16                                    Pt            --         --      0.08  0.18                                    Ag            --         --      --    0.10                                    ______________________________________                                    

                  TABLE V                                                          ______________________________________                                         Substrate: n-type GaAs                                                         Au       Pt       Be       Ag     Cu     Al                                    Au    --     0.04     0.09   0.02   0.08   0.10                                Pt    --     --       0.05   0.02   0.04   0.06                                Be    --     --       --     0.09   0.01   0.01                                Ag    --     --       --     --     0.06   0.08                                Cu    --     --       --     --     --     0.02                                ______________________________________                                    

                  TABLE VI                                                         ______________________________________                                         Substrate: p-type GaAs                                                                       Au        Al                                                     Au              --          0.08                                               ______________________________________                                    

                  TABLE VII                                                        ______________________________________                                         Substrate: n-type GaP                                                          Cu    Al      Au        Pt       Mg      Ag                                    Cu  --    0.15    0.08    0.2      0.16    --                                  Al  --    --      0.2     0.4      0.01    0.15                                Au  --    --      --       0.17    0.2     0.08                                Pt  --    --      --      --       0.4     0.2                                 Mg  --    --      --      --       --      0.16                                ______________________________________                                    

                  TABLE VIII                                                       ______________________________________                                         Substrate: n-type Ge                                                                         Au        Al                                                     Au              --          0.03                                               ______________________________________                                    

                  TABLE IX                                                         ______________________________________                                         Substrate: n-type Inp                                                                        Au        Ag                                                     Au              --          0.05                                               ______________________________________                                    

                  TABLE X                                                          ______________________________________                                         Substrate: n-type PbO                                                                      Ag   Bi      Ni      Pb    In                                      Ag            --     0.01    0.01  --    0.02                                  Bi            --     --      0.02  0.01  0.01                                  Ni            --     --      --    0.01  0.03                                  Pb            --     --      --    --    0.02                                  ______________________________________                                    

                  TABLE XI                                                         ______________________________________                                         Substrate: n-type Si                                                                         Au     Mo      PtSi    W                                         Au              --       0.2     0.07  0.13                                    Mo              --       --      0.3   0.09                                    PtSi            --       --      --    0.2                                     ______________________________________                                    

                  TABLE XII                                                        ______________________________________                                         Substrate: p-type Si                                                                         Au        PtSi                                                   Au              --          0.05                                               ______________________________________                                    

                  TABLE XIII                                                       ______________________________________                                         Substrate: n-type SiC                                                                        Au        Al                                                     Au              --          0.05                                               ______________________________________                                    

                  TABLE XIV                                                        ______________________________________                                         Substrate: n-type S.sub.n O.sub.2                                                      Au        Ag             Cu                                            Au        --          0.3            0.5                                       Ag        --          --             0.18                                      ______________________________________                                    

                  TABLE XV                                                         ______________________________________                                         Substrate: n-type ZnO                                                                      Au       Pt      Pd      Ag                                        Au            --         0.10    0.03  0.03                                    Pt            --         --      0.07  0.07                                    ______________________________________                                    

                  TABLE XVI                                                        ______________________________________                                         Substrate: n-type ZnS                                                          Au    Pd      Pt      Cu    Ag    In   Al  Ti   Mg                             Au  --    0.13    0.16  0.2   0.4   0.5  1.2 0.9  1.2                          Pd  --    --      0.03  0.12  0.2   0.4  1.1 0.8  1.0                          Pt        --      --    0.09  0.19  0.3  1.0 0.7  1.0                          Cu  --    --      --    --    0.10  0.2  1.0 0.6  0.9                          Ag  --    --      --    --    --     0.15                                                                               0.8 0.6  0.8                          In  --    --      --    --    --    --   0.7 0.4  0.7                          Al  --    --      --    --    --    --   --  0.3   0.02                        Ti  --    --      --    --    --    --   --  --   0.3                          ______________________________________                                    

                  TABLE XVII                                                       ______________________________________                                         Substrate: n-type ZnSe                                                                     Au       pt      Cu      Mg                                        Au            --         0.04    0.26  0.7                                     Pt            --         --      0.3   0.7                                     Cu            --         --      --    0.4                                     ______________________________________                                    

                  TABLE XVIII                                                      ______________________________________                                         Substrate: n-type ZnO                                                                        Cu        In       Ti                                            Cu              --          0.15     0.15                                      In              --          --       --                                        ______________________________________                                     

We claim:
 1. A voltage reference comprising:first and second field effect devices, each having a source, a drain and a gate, and each device having a different gate-to-channel potential barrier characteristic from the other; means coupled to the first and second field-effect devices for producing substantially identical current flow to the drains of the first and second field-effect devices; means coupled to the first and second field-effect devices for sensing the voltage difference between the gate-to-source voltage of the first field-effect device and the gate-to-source voltage of the second field-effect device; and means coupled to each of said field-effect devices for producing substantially identical drain-to-source voltages for said first and second field-effect devices.
 2. A voltage reference as in claim 1 wherein the first field-effect device has a first substrate and utilizes a first metal in combination with the first substrate as its gate, the second field-effect device has a second substrate substantially identical to the first substrate and utilizes a second metal in combination with the second substrate as its gate and each of the metals make rectifying contacts to the respective substrates of the first and second-field effect devices.
 3. A voltage reference as in claim 2 wherein the first and second field-effect devices are formed on the same substrate.
 4. A voltage reference as in claim 2 wherein the first and second substrates are n-type AlAs and the first and second metals are selected from the group consisting of Au and Pt, the first and second metals being different.
 5. A voltage reference as in claim 2 wherein the first and second substrates are n-type CdS and the first and second metals are selected from the group consisting of Pt, Au, Pd, Cu, Ag and Ni, the first and second metals being different.
 6. A voltage reference as in claim 2 wherein the first and second substrates are n-type CdSe and the first and second metals are selected from the group consisting of Pt, Au, Ag, and Cu, the first and second metals being different.
 7. A voltage reference as in claim 2 wherein the first and second substrates are n-type CdTe and the first and second metals are selected from the group consisting of Au, Pt, Ag, and Al, the first and second metals being different.
 8. A voltage reference as in claim 2 wherein the first and second substrates are n-type GaAs and the first and second metals are selected from a group consisting of Au, Pt, Be, Ag, Cu, and Al, the first and second metals being different.
 9. A voltage reference as in claim 2 wherein the first and second substrates are p-type GaAs and the first and second metals are selected from the group consisting of Au and Al, the first and second metals being different.
 10. A voltage reference as in claim 2 wherein the first and second substrates are n-type GaP and the first and second metals are selected from the group consisting of Cu, Al, Au, Pt, Mg, and Ag, the first and second metals being different.
 11. A voltage reference as in claim 2 wherein the first and second substrates are n-type Ge and the first and second metals are selected from the group consisting of Au and Al, the first and second metals being different.
 12. A voltage reference as in claim 2 wherein the first and second substrates are n-type InP and the first and second metals are selected from the group consisting of Au and Ag, the first and second metals being different.
 13. A voltage reference as in claim 2 wherein the first and second substrates are n-type PbO and the first and second metals are selected from the group consisting of Ag, Bi, Ni, Pb, and In, the first and second metals being different.
 14. A voltage reference as in claim 2 wherein the first and second substrates are n-type Si and the first and second metals are selected from the group consisting of Au, mo, PtSi, and W, the first and second metals being different.
 15. A voltage reference as in claim 2 wherein the first and second substrates are p-type Si and the first and second metals are selected from the group consisting of Au and PtSi, the first and second metals being different.
 16. A voltage reference as in claim 2 wherein the first and second substrates are n-type SiC, the first metal is Au and the second metal is Al.
 17. A voltage reference as in claim 2 wherein the first and second substrates are n-type SnO₂ and the first and second metals are selected from the group consisting of Au, Ag, and Cu, the first and second metals being different.
 18. A voltage reference as in claim 2 wherein the first and second substrates are n-type ZnO and the first and second metals are selected from the group consisting of Au, Pt, Pd, and Ag, the first and second metals being different.
 19. A voltage reference as in claim 2 wherein the first and second substrates are n-type ZnS and the first and second metals are selected from the group consisting of Au, Pd, Pt, Cu, Ag, In, Al, Ti, and Mg, the first and second metals being different.
 20. A voltage reference as in claim 2 wherein the first and second substrates are n-type ZnSe and the first and second metals are selected from the group consisting of Au, Pt, Cu, and Mg, the first and second metals being different.
 21. A voltage reference as in claim 2 wherein the first substrate is selected from the group consisting of n-type AlAs, n-type CdS, n-type CdSe, n-type CdTe, n-type GaAs, p-type GaAs, n-type GaP, n-type Ge, n-type InP, n-type PbO, n-type Si, p-type Si, n-type SiC, n-type SnO₂, n-type ZnO, n-type ZnS, and n-type ZnSe.
 22. A voltage reference as in claim 2 wherein the first and second substrates are n-type ZnO, the first metal is Cu and the second metal is selected from the group consisting of In and Ti.
 23. A voltage reference comprising:first and second field-effect devices, each having a source contact, a drain contact and a gate contact and each device having a different gate-to-channel potential barrier characteristic from the other; means connected to the drain contacts of the first and second field-effect devices for supplying current thereto; means for coupling the source contacts of the first and second field-effect devices to the gate contact of the first field-effect device; an amplifier having a non-inverting input coupled to the source contact of the first field-effect device, an inverting input coupled to the source contact of the second field-effect device and an output coupled to the gate contact of the second field-effect device; and means for coupling to the gates of the first and second field-effect devices.
 24. A voltage reference as in claim 23 wherein means for coupling comprise:a first resistor having a first terminal coupled to the first field-effect device source contact and the non-inverting input of the amplifier and having a second terminal coupled to the first field-effect gate contact; and a second resistor having a first terminal coupled to the second field-effect device source contact and the inverting input of the amplifier and having a second terminal coupled to the first resistor's second terminal.
 25. A voltage reference as in claim 24 wherein means for coupling comprise:a third field-effect device having a drain contact coupled to the source contact of the first field-effect device, a gate contact coupled to the gate contact of the first field-effect device, and a source contact coupled to the gate contact of the first field-effect device; and a fourth field-effect device having a drain contact coupled to the second field-effect device source contact, a gate contact coupled to the first field-effect device gate contact, and a source contact coupled to the first field-effect device gate contact.
 26. A voltage reference comprising:first and second voltage variable capacitors, each having a gate and a substrate and each capacitor having a different gate-to-substrate potential barrier characteristic from the other; means coupled to the first and second voltage variable capacitors for producing substantially the same depletion layer width in the first and second voltage variable capacitors; and means for sensing the voltage difference between the gate-to-substrate voltages produced by the first and second voltage variable capacitors.
 27. A voltage reference as in claim 26 wherein the first and second voltage variable capacitors are formed on the same substrate.
 28. A voltage reference as in claim 26 wherein the first voltage variable capacitor utilizes a first metal for its gate, the second voltage variable capacitor utilizes a second metal for its gate and each of the metals make rectifying contacts to the respective substrates of the first and second voltage variable capacitors.
 29. A voltage reference as in claim 28 wherein the first and second voltage variable capacitors are formed on the same substrate.
 30. A voltage reference as in claim 28 wherein the first and second substrates are n-type CdS and the first and second metals are selected from the group consisting of Pt, Au, Pd, Cu, Ni, and Ag, the first and second metals being different.
 31. A voltage reference as in claim 28 wherein the first and second substrates are n-type CdSe and the first and second metals are selected from the group consisting of Pt, Au, Ag, and Cu, the first and second metals being different.
 32. A voltage reference as in claim 28 wherein the first and second substrates are n-type CdTe and the first and second metals are selected from the group consisting of Au, Pt, Ag, and Al, the first and second metals being different.
 33. A voltage reference as in claim 28 wherein the first and second substrates are n-type GaAs and the first and second metals are selected from a group consisting of Au, Pt, Be, Ag, Cu, and Al, the first and second metals being different.
 34. A voltage reference as in claim 28 wherein the first and second substrates are p-type GaAs and the first and second metals are selected from the group consisting of Au and Al, the first and second metals being different.
 35. A voltage reference as in claim 28 wherein the first and second substrates are n-type GaP and the first and second metals are selected from the group consisting of Cu, Al, Au, Pt, Mg, and Ag, the first and second metals being different.
 36. A voltage reference as in claim 28 wherein the first and second substrates are n-type Ge and the first and second metals are selected from the group consisting of Au and Al, the first and second metals being different.
 37. A voltage reference as in claim 28 wherein the first and second substrates are n-type InP and the first and second metals are selected from the group consisting of Au and Ag, the first and second metals being different.
 38. A voltage reference as in claim 28 wherein the first and second substrates are n-type PbO and the first and second metals are selected from the group consisting of Ag, Bi, Ni, Pb, and In, the first and second metals being different.
 39. A voltage reference as in claim 28 wherein the first and second substrates are n-type Si and the first and second metals are selected from the group consisting of Au, Mo, PtSi, and W, the first and second metals being different.
 40. A voltage reference as in claim 28 wherein the first and second substrates are p-type Si and the first and second metals are selected from the group consisting of Au and PtSi, the first and second metals being different.
 41. A voltage reference as in claim 28 wherein the first and second substrates are n-type SiC, the first metal is Au and the second metal is Al.
 42. A voltage reference as in claim 28 wherein the first and second substrates are n-type SnO₂ and the first and second metals are selected from the group consisting of Au, Ag, and Cu, the first and second metals being different.
 43. A voltage reference as in claim 28 wherein the first and second substrates are n-type ZnO and the first and second metals are selected from the group consisting of Au, Pt, Pd, and Ag, the first and second metals being different.
 44. A voltage reference as in claim 28 wherein the first and second substrates are n-type ZnS and the first and second metals are selected from the group consisting of Au, Pd, Pt, Cu, Ag, In, Al, Ti, and Mg, the first and second metals being different.
 45. A voltage reference as in claim 28 wherein the first and second substrates are n-type ZnSe and the first and second metals are selected from the group consisting of Au, Pt, Cu, and Mg, the first and second metals being different.
 46. A voltage reference as in claim 28 wherein the first substrate is selected from the group consisting of n-type AlAs, n-type CdS, n-type CdSe, n-type CdTe, n-type GaAs, p-type GaAs, n-type GaP, n-type Ge, n-type InP, n-type PbO, n-type Si, p-type Si, n-type SiC, n-type SnO₂, n-type ZnO, n-type ZnS, and n-type ZnSe.
 47. A voltage reference as in claim 28 wherein the first and second substrates are n-type AlAs and the first metal is Au and the second metal is Pt.
 48. A voltage reference as in claim 28 wherein the first and second substrates are n-type ZnO, the first metal is Cu and the second metal is selected from the group consisting of In and Ti.
 49. A voltage reference as in claim 28 wherein the means for producing substantially the same depletion layer width in the first and second voltage variable capacitors comprise:a first means for rectifying and filtering; second means for rectifying and filtering; an amplifier having a first input coupled to the first means, a second input coupled to the second means, and an output coupled to the first voltage variable capacitor; an alternating voltage source; a first capacitor coupled to the alternating voltage source and the first voltage variable capacitor; a second capacitor coupled to the alternating voltage source and the second voltage variable capacitor; a ground; a first resistor coupled between the first voltage variable capacitor and the ground; a second resistor coupled between the second voltage variable capacitor and the ground; and a bias source coupled between the second voltage variable capacitor and the ground.
 50. A voltage reference as in claim 28 wherein the means for producing substantially the same depletion layer width in the first and second voltage variable capacitors comprise:first means having an input coupled to the second voltage variable capacitor and an output for rectifying and filtering; second means having an input coupled to the second voltage variable capacitor and an output for rectifying and filtering; an amplifier having a first input coupled to the first means output, a second input coupled to the second means output, and an output coupled to the first means input; a first alternating current source coupled to the first means input; a second alternating current source coupled to the second means input; and a bias source coupled across the second voltage variable capacitor.
 51. A method for producing a reference voltage comprising the steps of:producing different gate-to-channel potential barrier characteristics in a plurality of field-effect devices, each field-effect device having a gate, a source and a drain; biasing the plurality of field-effect devices to carry substantially identical drain current flow; maintaining substantially similar drain to substrate potentials for said plurality of field-effect devices; and sensing the voltage difference between the gate-to-source voltages produced by a plurality of said plurality of field-effect devices. 